Conductive polymer compositions exhibiting positive temperature coefficient (PTC) behavior, and electrical devices containing these compositions, are well-known, as disclosed, for example, in U.S. Pat. Nos. 2,952,761; 2,978,665; 3,243,753; 3,351,882; 3,571,777; 3,697,450; 3,757,086; 3,760,495; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,950,604; 4,017,715; 4,072,848; 4,085,286; 4,117,312; 4,124,747; 4,177,376; 4,177,446; 4,188,276; 4,237,441; 4,238,812; 4,242,573; 4,246,468; 4,250,400; 4,252,692; 4,255,698; 4,271,350; 4,272,471; 4,304,987; 4,309,596; 4,309,597; 4,314,230; 4,314,231; 4,315,237; 4,317,027; 4,318,881; 4,318,220; 4,327,351; 4,329,726; 4,330,704; 4,334,148; 4,334,351; 4,352,083; 4,361,799; 4,388,607; 4,398,084; 4,400,614; 4,413,301; 4,425,397; 4,426,339; 4,426,633; 4,427,877; 4,435,639; 4,429,216; 4,442,139; 4,459,473; 4,481,498; 4,476,450; 4,502,929; 4,514,620; 4,517,449; 4,534,889; 4,545,926; 4,591,700; 4,724,417; 4,743,321; 4,764,664; 4,845,838; 4,857,880; German OLS No. 1,634,999; German OLS No. 2,746,602; German OLS No. 2,821,799; European Application No. 38,718; European Application No. 38,718; European Application No. 38,713; European Application No. 38,714; UK Application No. 2,076,106A; European Application No. 63,440; European Application No. 74,281; European Application No. 92,406; European Application No. 119,807; European Application No. 84,304,502.2; European Application No. 84,305,584.7; European Application No. 84,307,984.9; UK Patent Nos. 1,470,502 and 1,470,503; Klason and Kubat, J. Applied Polymer Science, 19, 831-845 (1975); J. Meyer, Polymer Engineering and Science, vol. 13 No. 6, 462-468 (1973); J. Meyer, Polymer Engineering and Science, vol. 14, No. 10, 706-716 (1974); M. Narkis, Polymer Engineering and Science, vol. 18, No. 8, 649-653 (1978); and M. Narkis, J. Applied Polymer Science, vol. 25, 1515-1518 (1980). The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference. In particular, J. Meyer, M. Narkis and U.S. Pat. No. 4,237,441 disclose conductive polymer compositions containing various conventional carbon blacks and their PTC behaviors.
It is known that polymers, including crystalline polymers, can be made electrically conductive by dispersing therein suitable amounts of finely divided conductive fillers. Some conductive polymers exhibit what is known as PTC (positive temperature coefficient) behavior. As used herein, the terms "PTC polymer," "composition exhibiting PTC behavior" and "PTC composition" are used to denote a composition which has an R.sub.14 value of at least 2.5 and an R.sub.100 value of at least 10, and preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of a 14.degree. C. range, R.sub.100 is the ratio of the resistivities at the end and the beginning of a 100.degree. C. range, and R.sub.30 is the ratio of the resistivities at the end and the beginning of a 30.degree. C. range. A plot of the log of the resistance of a PTC element (i.e., an element composed of a PTC composition) against temperature will often show a sharp change in slope over a part of the temperature range in which the composition has an R.sub.100 value of at least 10. The term "switching temperature" (abbreviated herein to T.sub.s) is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie either side of the portion showing the sharp change in slope. The term "peak resistivity" is used herein to denote the maximum resistivity which the composition exhibits above T.sub.s, and the term "peak temperature" is used to denote the temperature at which the composition has its peak resistivity. This relationship is illustrated in FIG. 6, where A indicates resistivity at 25.degree. C., B is the average slope of the curve and C indicates maximum resistivity.
It is well known that PTC behavior of conductive polymer compositions depends on the physical and chemical properties of the polymer and carbon black which are mixed and dispersed. Recently, the demand for circuit protection devices having a high breakdown voltage has increased for use in electrical devices consuming large amounts of electric power. In many applications, the circuit protection device should withstand a high power supply voltage when the circuit is in a fault state , that is, when the circuit protection device has been tripped to its high resistance state. It is understood that the composition should be an insulator at the tripped temperature, and that the conductive polymer composition should exhibit PTC behavior.
A circuit protection device has a relatively low resistance under normal conditions, but under fault conditions it converts to high resistance, i.e., is "tripped" above the switching temperature of the PTC polymer to reduce the current flow through the device and the circuit it protects and thereby protect the circuit. Recent electrical or electronic devices and apparatuses powered by a battery supplying a large amount of power have relatively low resistance, sometimes, very low resistance. Particularly, the use of devices having very low resistance has rapidly increased with development of electrical and electronic technology, and therefore the demand for protection circuit devices having very low resistance, e.g., at most about 500 m.OMEGA., but able to withstand high power supply voltages has remarkably increased.
In circuit protection devices, the requirement of very low resistance in normal operation requires that the contact resistance between electrodes of the circuit protection device and the PTC polymer in contact with the electrodes be reduced, and that the PTC polymer layer between the electrodes itself be made as thin as possible. However, very thin conventional PTC polymer elements with low resistance are usable only at relatively low power supply voltages because, in general, the breakdown voltage of the device decreases as the thickness of the polymer layer or the resistivity of the polymer material (and hence the resistance of the device) is decreased. As defined herein, the term "breakdown voltage" is the maximum power supply voltage, increasing at a steady rate of 60 V/min, which can be applied across the PTC polymer element without causing dielectric breakdown of the PTC polymer composition.
High breakdown voltage is often required since a relatively high power supply voltage is needed to drive devices such as a motor. However, although a conventional circuit protection device containing a low resistivity conductive polymer composition allows more current to pass through, often these devices cannot simultaneously withstand a high voltage.
Two-step irradiation cross-linking and heating between the two crosslinking steps are disclosed in U.S. Pat. Nos. 4,857,880 and 4,724,417 to maintain PTC behavior after frequent or long tripping and high voltage. However, this requires at least three processes and is complicated and expensive.
Conventional PTC conductive polymer compositions containing various types of carbon blacks do not have remarkably high breakdown voltage. However, we have discovered that a composition comprising certain carbon blacks has a remarkably high breakdown voltage despite having very low resistance and excellent PTC behavior.